1. Field of the Invention
The present invention relates to an antiferroelectric liquid crystal display device and a apparatus, and methods of driving the same.
2. Description of the Related Art
In recent years, an antiferroelectric liquid crystal display device having a larger field angle and a higher response than those of a conventional TN type liquid crystal display device has received a great deal of attention.
Ferroelectric liquid crystal display devices are classified into a ferroelectric liquid crystal display device using a ferroelectric liquid crystal exhibiting bistable characteristics (bistable states) in the aligned states of liquid crystal molecules, and an antiferroelectric liquid crystal display device using an antiferroelectric liquid crystal exhibiting tristable characteristics in the aligned states of liquid crystal molecules. Extensive studies have been recently made on antiferroelectric liquid crystal display devices.
The antiferroelectric liquid crystal display device exhibits tristable characteristics in the aligned states of the liquid crystal molecules. When a voltage exceeding the first threshold value is applied to the element, the liquid crystal is oriented in the first or second ferroelectric phase in which the molecules align in the first or the second direction, in accordance with the polarity of the voltage applied to the element. When a voltage having a value smaller than the second threshold value smaller than the first threshold value is applied to the antiferroelectric liquid crystal display device, the liquid crystal is aligned in an antiferroelectric phase in which the molecules point in an intermediate direction between the first and second directions. For this reason, the transmission axes of a pair of polarizing plates located on both sides of the liquid crystal display device are set with reference to the optical axis of the antiferroelectric phase to control the light transmittance, thereby displaying an image.
The antiferroelectric liquid crystal has a memory function (bistable states) of maintaining the aligned state in the first or second ferroelectric phase or the antiferroelectric phase regardless of changes in applied voltage to the liquid crystal if this voltage falls within the range between the first and second threshold values. A conventional antiferroelectric liquid crystal display device utilizes this memory function of the aligned state and is therefore driven in accordance with a simple matrix scheme.
The memory function of the aligned state of the antiferroelectric liquid crystal is enhanced as the difference between the first and second threshold values is larger. For this reason, the conventional antiferroelectric liquid crystal display device driven by the simple matrix scheme uses an antiferroelectric liquid crystal having a large difference between the first and second threshold values.
In the liquid crystal display device using the liquid crystal having a large difference between the first and second threshold values, a drive voltage for the liquid crystal display device is inevitably high. For this reason, it is difficult to drive the conventional antiferroelectric liquid crystal display device, using a conventional liquid crystal display device drive LSI.
The antiferroelectric liquid crystal has a high response speed in alignment from the antiferroelectric phase to the first or second ferroelectric phase. However, it has a low response speed in alignment from the first or second ferroelectric phase to the antiferroelectric phase. For this reason, when liquid crystal molecules are to be aligned in the antiferroelectric phase in the liquid crystal display device of a simple matrix type, the selection period may end before the liquid crystal is perfectly aligned in the antiferroelectric phase. If this occurs, the contrast of the display image is lowered. When the selection period is prolonged to prevent the above trouble, it is difficult to drive the antiferroelectric liquid crystal display device at a high duty ratio.
The voltage vs. transmittance characteristics of the conventional antiferroelectric liquid crystal display device have a large hysteresis, as shown in FIG. 1. In the graph of FIG. 1, the maximum transmittance of the liquid crystal display device is defined as 100%, and the minimum transmittance is defined as 0%. When a large hysteresis is present in the voltage vs. transmittance characteristics, the transmittance of the antiferroelectric liquid crystal display device greatly varies even if a voltage applied to the element is kept unchanged. For this reason, it is difficult to control the transmittance so as to perform gradation display.